1. Field of the Invention.
The present invention relates to time domain noise rejection. In particular, the present invention relates to noise rejection in a sonic distance measuring transducer.
2. Description of the Prior Art.
Delay lines in sonic pulses have been utilized to measure distances. A linear distance measuring device employing a sonic transducer is disclosed in Tellerman U.S. Pat. No. 3,898,555. In this device, a mode converter is placed at one end of a ferromagnetic waveguide. A movable permanent magnet is mounted on the waveguide. Circuitry is provided for applying electrical pulses, from a pulse generator to a wire extending through the waveguide. When the magnetic field generated by an electrical pulse interacts with the magnetic field of the magnet, a sonic torsional pulse is launched for transmission along the waveguide. The sonic torsional pulse is sensed by the mode converter, which in turn generates an electrical return signal that is sensed and used to terminate a DC output pulse that is started at the time the pulse is applied to the wire and waveguide, is produced each pulse representing the interval between application of a pulse to the wire and reception of the corresponding electrical signal from the mode converter. The pulse train is averaged (filtered) to provide a DC voltage output proportional to the value of the pulse train which in turn is directly proportional to the position of the permanent magnet along the waveguide.
The present invention insures that the output pulses are ended only when the signal sensed is from the mode converter corresponding to the start pulse that initiated the DC output pulse. The frequency spectrum of the return signal is in the radio frequency band of industrial noise, such as that caused by switching contacts, solenoids, off-on transients of motors, etc. Such noise, induced either magnetically, capacitively or by radiant field, is picked up as a signal in the transducer. The noise on the return line may falsely terminate the DC output pulse thus inducing transient changes in the DC pulse train.
Conventional methods to prevent reception of noise have proved inadequate. Because the noise is in the same frequency band as the expected return signal, normal passive selective band pass filters are not effective. Attempts to raise the return signal level from the mode converter to improve the signal-to-noise ratio so that noise can be rejected have also proved unreliable because the amplitudes of mode converter return signal cannot be raised significantly higher than the noise amplitude using normal circuitry and manufacturable transducer parts.